1. Field of the Invention
This invention relates to magnetoresistors, and more particularly to Corbino disk devices.
2. Description of the Related Art
Magnetoresistors exhibit a resistance which varies in accordance with an applied vertical magnetic field. Most magnetoresistors exhibit positive magnetoresistance, with the resistance increasing in response to increases in magnetic field strength. The basis of the magnetoresistance is the Lorentz force, which causes the electrons to move in curved paths between collisions. This increases the effective electron path length, and is reflected as an increase in resistance.
As the electrons are reflected towards one side of the magnetoresistor (MR), the accumulation of electrons along that side produces an internal transverse electric field which opposes and tends to negate the effect of the externally applied magnetic field. This phenomenon is referred to as the Hall effect, and is described for example in Van Nostrand's Scientific Encyclopedia, 7th Edition, ed. by D. M. Considine, Van Nostrand Reinhold, 1989, page 1398.
By shorting the Hall electric field, one can obtain a magnetoresistance which does not saturate. One approach to accomplishing this shorting effect is to form a pattern of transverse shorting strips across an elongate MR bar. The conductive shorting strips form contacts with the underlying MR material, and in effect short circuit the Hall fields that would otherwise be built up. Examples of such Hall shorting strips are given in Wang et al., "Semiconductive Magnetoresistors", MRL Bulletin of Research and Development, Vol. 2, No. 2, September 1988, and in U.S. Pat. Nos. 3,772,100, 3,852,103 and 3,898,359.
Another approach to eliminating the Hall field is to choose an MR geometry such that the field does not exist. A Corbino disk is a well-known device of this type. Corbino disks are circular disks of MR material, with a conductive terminal at the hub or center of the disk, and a conductive ring encircling the hub terminal. The ring and hub terminal are formed on the same side of the disk, or can extend through the disk. The circularly symmetric configuration of the conductive ring perpendicular to the current flow prevents any local accumulations of charge, and thereby inhibits the development of a Hall field. Corbino disks are discussed, for example, in H.H. Wieder, "Hall Generators and Magnetoresistors", Pion Ltd., 1970, pages 1-6.
While Corbino disks tend to be more sensitive than elongate MR bars for a given area, MR bars have the advantage that they have layout flexibility. For example, long linear MRs can be designed with their total resistance easily selected by choosing appropriate dimensions for the bar. For certain applications, such as in automobiles where high accuracy is required and operating temperatures can reach 240.degree. C., a greater degree of sensitivity than that provided by either of the present devices would be desirable, but it would still be useful to be able to easily tailor the dimensions of the MR device to adjust its total resistance and to provide a layout that fits the requirement.